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Postings from September 2000
On Thu, 14 Sep 2000, Andy Dougherty wrote:
> Do you mean that the following program might not print '5' (well, about 5,
> given sleep's uncertaintites ...)
>
> #include <stdio.h>
> #include <sys/types.h>
> #include <time.h> /* May need sys/time.h instead */
> int main(void)
> {
> time_t start, stop;
> start = time((time_t *) 0);
> sleep(5);
> stop = time((time_t *) 0);
> return printf("The difference is %ld seconds.\n", stop - start);
> }
[snip]
> I agree those are portability bugs in those modules and apps. Just like
> assumptions that '/' is the directory separator, and that integers are 32
> bits long, and many of the other problems cited in perlport.pod.
You have another assumption up there: that time_t == signed long (since
you're printing it with %ld) with a resolution of seconds (since you say
"%ld seconds"). ISO/IEC 9899:1999 (draft C standard, the only C standard-y
thing I have around) says that time_t is an "arithmetic type" (OK, so
you're allowed to subtract one from another) "capable of representing
time", and that "the range and precision representable in clock_t and
time_t are implementation-defined". So subtracting them may not give you
seconds since any epoch (that's what the difftime() is for; it explicitly
gives you seconds of difference, as a double), and even if you knew it
did, there's no guarantee that %ld is the correct format to print it out.
Might as well be %llu or %d as %ld.
Maybe POSIX makes more guarantees.
Cheers,
Philip
--
Philip Newton <newton@newton.digitalspace.net>
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